The United States Postal Service employs many types of letter sorting machines. Some examples are bar code sorters, optical character readers, multiple position letter sorters and delivery bar code sorters. Such letter sorting machines are well known to those skilled in the art, and these machines will therefore not be described in greater detail.
The operation of these machines is currently quite labor intensive. Letters are generally conveyed to and from letter sorting machines in trays which are in turn conveyed in relatively large carts that are moved by hand. This requires a significant amount of labor, and the carts take up a significant amount of floor space. Two-pass delivery bar code sorters also require staging or storing of letters between passes. Such staging is currently done with the same trays and carts, resulting in the same disadvantages.
Many letter sorting machines are modular, i.e., their capacity can be increased or decreased by adding or removing modular units.